Self-latching mechanisms for holding a cylindrical item are well known. This is not surprising since cylindrical shapes are common as either conduits or as structural elements. The cylindrical shape is common due to its ease of manufacture and favorable mechanical properties.
Typically, the self-latching mechanisms are adapted to grab and hold the cylindrical item to prevent gravity induced downward movement of the cylindrical item. The self-latching mechanisms are adapted to allow the user to release the cylindrical item from the self-latching mechanism. Examples of such self-latching mechanisms include U.S. Pat. No. 836,303 granted on Nov. 20, 1906, to Christensen.
The Christensen patent describes an automatic locking collar which may be adjusted either up or down on a cylindrical item. The automatic locking collar will lock automatically in any position against further movement in one direction on the cylindrical item. The Christensen patent also teaches that the collar may be fixed and the cylindrical item may be moved up and down at will through the collar until the collar locks automatically against further movement of the cylindrical item in one direction.
The Christensen locking mechanism is a latch plate with a cut-out slightly larger than the cylindrical item. When the latch plate is released, the latch plate rotates down and the inside edge of the cut-out in the latch plate contacts the cylindrical item. If the collar is fixed, the force of gravity moves the cylindrical item downward and causes the latch plate to rotate around an axis in order to move the contacting edge of the latch plate further toward the center of the cylindrical item. Since the latch plate and the cylindrical item cannot occupy the same space, the downward movement of the cylindrical item continues briefly until the interference between the latch plate and the cylindrical item binds the cylindrical item in place.
In order to free or release the cylindrical item, the latch plate is rotated about its pivot so as to disengage from the cylindrical item. The cylindrical item is released when the latch plate is rotated up and out of the way, in order to free the cylindrical item. The latch plate in many self-latching mechanisms can be rotated out of the way either directly through use of a handle, or indirectly by lifting the cylindrical item up to push the latch plate up. Since such a latch plate is basically a one-way brake, by reason of the servo action of its engagement, a single latch plate will be used to prevent motion in only one direction.
The prior art contains a wide variety of variations on the self-latching mechanism in a variety of applications.
U.S. Pat. No. 1,494,022, issued on May 13, 1924, to Roberson teaches a device for clamping a sucker rod to hold the rod while it is being assembled and inserted into a well. The Roberson device uses a latch plate which is mounted on a hinged plate that straddles the sucker rod as it passes through a cylindrical void slightly larger than the sucker rod. The hinged plate can be rotated so the latch plate does not interfere with the movement of the sucker rod. The hinged plate can also be rotated toward the sucker rod so that contact with the latch plate will bind the sucker rod against the inner wall of the cylindrical void.
U.S. Pat. No. 2,051,969, issued on Aug. 25, 1936, to Shastock is directed to an automatic tube adjuster for telescoping tubes. The Shastock device uses a latch plate with a cut-out through which the cylindrical item passes. The Shastock device uses a spring to automatically move the latch plate into contact with the cylindrical item in lieu of using the weight of the handle to rotate the latch plate into position.
Self-latching mechanisms have been used since at least 1906 in a wide variety of applications, and yet there is a significant limitation in such prior art mechanisms. The limitation is that the prior art mechanisms provide only limited control over the motion of the cylindrical item relative to the self-latching mechanism. The bind and release states of the latch are two polar extremes. Either the cylindrical item is totally bound and cannot move at all, or the cylindrical item is totally released and thus free to fall through the latch mechanism without any impediment to travel.
This shortcoming in prior art devices is not serious if the use is one in which the user is holding the cylindrical item to position it. If the user is holding the cylindrical item, it cannot fall when the latch plate is released. Examples include precise positioning of a dress form to a selected height, or positioning the tilt of a drafting table. Once such adjustments are made, readjustments are not required until there is a new use for the dress form or the drafting table. In such circumstances it is not unduly burdensome to require the user to momentarily support the weight of the cylindrical item and any load attached to the cylindrical item.
Other applications for self-latching mechanisms may require more frequent adjustments. For example, a movie screen support will be set up and put away with each use. In this case the cylindrical object will typically have an extended position and a retracted position. While there may be some range of desired extended positions, the retracted position is fully retracted so the movie screen can be folded and stored. When the desired position of the retracted position is fully retracted, it is foreseeable that the user will release the latch mechanism and allow the cylindrical object to travel all the way back until it hits a mechanical stop that prevents further retraction. Since the latch mechanism lacks an intermediate state between fully latched and totally free, the cylindrical item if not manually restrained, accelerates under the force of gravity until hitting the mechanical stop.
The lack of intermediate states between being fully bound and being free to fall is not a problem if the latch mechanism is used in a device in which the cylindrical item can free fall without causing any harm. When the cylindrical item crashes into the mechanical stop, there will be noise; and the sudden movement and the noise may surprise the user. This situation is usually tolerable for most devices; and in some cases, such situations can be mitigated by adding springs or cushions to the mechanical stop to soften the impact and reduce the noise. However, as described below free fall is not suitable for all applications of latches.